The present invention pertains generally to a device for evaluating the stiffness of a golf shaft. More particularly, the present invention pertains to a device which uses stiffness measurements to locate the spine of a golf club shaft. The present invention is particularly, but not exclusively, useful as a device for finding the spine of a golf club shaft under static conditions and then determining the dynamic stiffness of the shaft at various angles about the shaft""s axis from the spine.
A typical golf club includes a head, a shaft and a grip. The head is attached to one end of the shaft and includes a face for contact with a golf ball. An elastomeric grip covers the other end of the shaft. Typically, the shaft is elongated and substantially cylindrical. Generally, the shaft is slightly tapered from a larger cross-section at the grip end to a smaller cross-section where the head is attached.
During the golf swing, the shaft is exposed to various forces. For example, because the face of the club head is not located on the axis of the shaft, the shaft is exposed to a torsional force when the club head impacts the ball. Moreover, a lateral or bending force is imparted to the shaft both at ball impact and during portions of the golf swing when the club head is accelerated or decelerated.
The actual amount of bend and twist experienced by the shaft during the golf swing will depend on the dimensions and construction of the shaft. Specifically, the flexibility (i.e. stiffness) of the shaft will determine the amount of bend and twist experienced by the shaft during the swing and at impact with the ball. As it happens, different golfers prefer shafts having different stiffnesses. For example, tour players that have an extremely fast swing speed generally prefer stiff shafts that will bend and twist very little during the swing. Because the amount of bend and twist is reduced, the head of the club is maintained at a proper alignment relative to the shaft resulting in a xe2x80x9csquarexe2x80x9d club face at impact. On the other hand, some players, such as women and seniors, often prefer flexible shafts. These players generally have a slower swing speed, and rely on a whipping action created by the flexible shaft to increase club head speed at ball impact. In any case, all golfers generally seek a set of clubs having little or no variation in shaft flexibility from club to club within the set.
Modern shafts are made of metal such as steel or composite materials such as carbon fiber (graphite) embedded in an epoxy matrix. During manufacture of both metal and composite shafts, a spine (sometimes called a seam) is generally created in the shaft. For example, to create a steel shaft, a flat shaft blank is typically rolled onto a tapered cylindrical mandrel and the edges of the blank are butt welded together. The weld creates a spine in the shaft that extends along the length of the shaft. Similarly, in shafts made from composite materials, fibers are generally woven into either a cloth-like material or a uni-directional tape and then impregnated with resin. In one technique, several pieces of impregnated cloth-like material or uni-directional tape are successively wrapped around a tapered cylindrical mandrel and then cured to form a composite shaft. Unfortunately, even when a substantial effort is expended to line up the ends of the successive pieces of cloth, overlaps and mismatches between the ends of adjacent pieces create a spine in the composite shaft.
Of important interest for the present invention, the spine affects the stiffness of the shaft. Specifically, the spine causes the stiffness of the shaft to vary when measured at different shaft angles. Stated another way, a different force is required to bend the shaft when the spine and axis of the shaft are in the plane of the bend than when the spine of the shaft is not in the bend plane. Importantly, this variation in stiffness can be used to locate the position of the spine of a shaft when the location of the spine is not visible. For purposes of the present disclosure, a spine plane can be defined as the plane containing both the axis of the shaft and the spine of the shaft. To locate the spine in this manner, the shaft is generally positioned horizontally with one end of the shaft clamped between two flat or xe2x80x9cvxe2x80x9d-shaped jaws. Then, a standard weight is attached to the second end of the shaft and the deflection of the second end is measured. Next, as the shaft is slowly rotated about the longitudinal axis of the shaft, a variation in the deflection of the second end becomes apparent. Since the spine is generally the stiffest part of the shaft, the spine location can be found by rotating the shaft until the deflection of the second end is at a minimum. Unfortunately, thin and fragile shafts are often damaged when clamped between two flat or xe2x80x9cvxe2x80x9d shaped jaws.
For purposes of the present disclosure, the technique for locating a spine described above is considered a static analysis since the second end of the shaft is essentially motionless when the deflection is measured. Once the position of the spine is located and marked, this information can be used in club building. Specifically, each club head can be oriented on the shaft with the spine positioned at a predetermined orientation relative to the club head face to thereby create a set of clubs having a consistent spine orientation and feel from club to club.
It is to be appreciated that when a player swings a golf club, the shaft does not always experience a simple static bend. In addition to the presence of a spine discussed above, other factors such as the orientation of fibers in a composite shaft or defects present in a metal or composite shaft can cause the stiffness of the shaft to vary with shaft angle. Unfortunately, static bend tests often fail to detect these defects. To more adequately model the dynamic golf swing and detect certain shaft defects, dynamic stiffness measurements can be used as a quality control tool to discard shafts having non-optimal dynamic stiffness.
After the spine has been located using the above described static analysis, dynamic stiffness measurements can be performed. To determine the dynamic stiffness of a shaft on various bend planes running through the shaft axis, one end of the shaft must be held. Next, the second end of the shaft is deflected and released to cause the second end of the shaft to oscillate within the original spine plane. The oscillation frequency is then measured as an indication of dynamic stiffness. The procedure can be repeated at different shaft angles. For example, the second end of the shaft can be deflected and released to oscillate in a plane normal to the original spine plane, and the oscillation frequency recorded. Once the dynamic stiffness at various shaft angles has been measured, the results can be used to discard shafts having dynamic stiffness values outside a predetermined range.
In light of the above it is an object of the present invention to provide a device suitable for the purposes of accurately determining the location of a golf club shaft spine and the dynamic stiffness of the shaft at various shaft angles relative to the spine. It is another object of the present invention to provide a device capable of performing both static and dynamic stiffness measurements on a golf club shaft without removing, realigning and regripping the shaft between tests. It is yet another object of the present invention to provide a device for measuring the stiffness of a golf club shaft having an improved clamping mechanism that firmly holds one end of the shaft during testing but does not damage thin and fragile shafts. Yet another object of the present invention is to provide a golf club evaluator which is easy to use, relatively simple to manufacture, and comparatively cost effective.
The present invention is directed to an apparatus for evaluating a golf club shaft. Specifically, the apparatus measures the static stiffness of the golf club shaft at radial increments to first locate the spine of the golf club shaft. Once the spine of the golf club shaft is located, the apparatus can determine the dynamic stiffness of the shaft at several radial orientations. For the present invention, the apparatus includes a clamp assembly and a flexing assembly. The assemblies are mounted on a common frame and distanced from each other to allow the clamp assembly to operate on one end of the golf club shaft and the flexing assembly to operate on the other end of the golf club shaft. The function of the clamp assembly is to hold one end of the golf club shaft and rotate the golf club shaft about the axis of the golf club shaft. The function of the flexing assembly is to deflect the head end of the golf club shaft and perform stiffness measurements.
In accordance with the present invention, the clamp assembly includes a tube, a first collet and a second collet. Preferably, the two collets are identical in construction and spaced apart along the length of the tube. The tube has a wall and is formed as an elongated, hollow cylinder that defines a tube axis along the longitudinal axis of the cylinder. For purposes of the present disclosure, the tube axis extends from the tube and through the flexing assembly. The wall of the tube is formed with an outer surface and an inner surface that surrounds the inside of the tube. Each collet has a set of six pins. To accommodate the pins of the first collet, six holes are formed in the wall of the tube. Preferably, the six holes are equally spaced around the circumference of the tube and located on a common plane. The common plane is preferably oriented normal to the tube axis.
Each pin is disposed in one of the holes formed in the tube, and extends into the tube radially through the wall of the tube. As such, each pin has a first end disposed within the tube for contact with the golf club shaft and a second end disposed outside of the tube. The first collet further includes a moveable collar positioned around the outside of the tube, and mounted on the tube. A recessed chamber is formed in the moveable collar facing the outer surface of the tube and extending around the circumference of the tube. The recessed chamber is formed with a conical surface that is distanced from the outer surface of the tube. The conical surface is beveled between a first end positioned relatively close to the outer surface of the tube and a second end positioned relatively far from the outer surface of the tube.
For the present invention, the second end of each pin extends into the recessed chamber and contacts the conical surface. Springs are provided to bias each pin away from the tube axis to thereby ensure the second end of each pin remains in contact with the conical surface at all times. This structure allows the moveable collar to be translated axially along the outside of the tube from a first position wherein a golf club shaft located within the tube is unclamped (released) to a second position wherein the golf club shaft within the tube is clamped. In the first position, the second end of each pin is in contact with the second end of the conical surface (the end farthest away from the outer surface of the tube).
During axial translation of the moveable collar along the outside of the tube from the first position to the second position, each pin is forced to translate radially into the tube, thereby creating a clamping force on a golf club shaft positioned in the tube. When the moveable collar is in the second position, the second end of each pin is in contact with the conical surface near the first end of the conical surface. To release the golf club shaft, the moveable collar can be axially translated along the outer surface of the tube from the second position to the first position. For the present invention, a motor driven lead screw can be used to translate the moveable collar axially along the outer surface of the tube. The second collet, which is constructed in the same manner as the first collet, is distanced from the first collet along the length of the tube to thereby allow the end of the golf club shaft to be clamped at two places (i.e. by two sets of pins).
A DC motor is provided for rotating the tube and collets about the tube axis. A belt or chain running around the tube can be driven by the DC motor to rotate the clamp assembly about the tube axis. Preferably, the DC motor is controlled by an electronic processor, such as a programmable logic controller (PLC). A desired rotation angle, xcex8, can be input into the PLC by the operator. The PLC can then cause the motor to rotate the tube and collets through the desired rotation angle, xcex8.
As indicated above, the function of the flexing assembly is to deflect the head end of the golf club shaft and perform stiffness measurements. The flexing assembly includes a robotic grab and release hand having two fingers. Each finger has a first end, second end and a midsection between the first end and the second end. Rollers for contacting the golf club shaft are attached to each finger at the finger""s first end.
The midsection of each finger is pivotally mounted to a plate, while the second end of each finger is attached to the shaft of a pneumatic cylinder. For the present invention, the pneumatic cylinder can be used to reconfigure the hand between a grab configuration and a release configuration. The rollers are juxtaposed in the grab configuration to allow the rollers to contact and hold the shaft during deflexion of the shaft. In the release configuration, the rollers are spaced apart to release the golf club shaft.
For the present invention, the robotic hand, plate and pneumatic cylinder are slideably mounted to a linear track. A pneumatic linear actuator is provided to move the hand, plate and cylinder along the linear track. With this cooperation of structure, the robotic hand, plate and cylinder move along a linear axis that is oriented at an angle of approximately 80 to 90 degrees from the tube axis. Thus, a bend plane containing both the linear axis and the tube axis is defined. Specifically, the hand, plate and cylinder are moveable along the linear axis to position the hand for grabbing the golf club shaft, and for subsequently deflecting the golf club shaft. The flexing assembly further includes a force gage such as a load cell to measure the force required to move the hand and thereby deflect the end of the golf club shaft through a predetermined distance.
For the present invention, a processor such as a PLC can be connected to a photo eye to measure the oscillation frequency of the end of the golf club shaft after the golf club shaft is deflected and released. Specifically, the photo eye can be positioned to pass a beam of light in a direction normal to the bend plane. More specifically, the light source can be positioned to pass the beam of light through the bend plane near the tube axis and into a reflector. Breaks in the reflected light beam can be used to calculate the oscillation frequency of the golf club shaft.
Adjustable limit sensors are provided to determine whether the oscillating golf club shaft travels outside of the vertical bend plane by a predetermined amount. Specifically, a photo eye can be positioned at the predetermined distance from the original bend plane and oriented to pass a beam of light in a direction parallel to the original bend plane. For the present invention, the distance between the light beam and the original bend plane can be adjusted by moving the photo eye. A reflector directs the light beam back to the photo eye. If the golf club shaft oscillates outside the bend plane by a predetermined amount, a break in the light beam will be detected by the photo eye for recordation by the processor. If desired, two adjustable limit sensors can be utilized, one on each side of the bend plane.
To use the apparatus to evaluate a golf club shaft, both the clamp assembly and the flexing assembly are first set up to receive the golf club shaft. Specifically, for the clamp assembly, the moveable collar is translated axially to the first position wherein the springs cause each pin to move away from the tube axis. Additionally, for the flexing assembly, the hand is configured into the release configuration wherein the rollers are separated. Next, the golf club shaft is inserted into the apparatus and clamped. In detail, one end of the golf club shaft is inserted into and through the tube until the other end of the golf club shaft is positioned in the tube. Then, one or both of the moveable collars are translated axially into their second positions wherein each pin is forced toward the tube axis to thereby clamp one end of the golf club shaft.
Once the golf club shaft is inserted into the apparatus and one end is clamped, the spine of the golf club shaft can be located. For this purpose, the robotic hand is first translated along the linear axis using the linear actuator until the midsection of each finger is positioned adjacent to the free end of the golf club shaft. Next, the hand is reconfigured into the grab configuration wherein the rollers are juxtaposed. Once the hand is in the grab configuration, the hand can be translated along the linear axis using the linear actuator until the rollers contact the head end of the golf club shaft. Upon contact between the rollers and the golf club shaft, the hand can be further translated along the linear axis to deflect the end of the golf club shaft to a holding point located at a predetermined distance from the tube axis. The force sensor indicates the force required to deflect the end of the golf club shaft to the holding point.
Next, while the end of the golf club shaft is deflected, the golf club shaft can be indexed through a full 360 degrees of rotation. Specifically, the collets and golf club shaft are incrementally rotated about the tube axis using the DC motor described above. The rollers on the fingers of the robotic hand allow the deflected end of the golf club shaft to rotate freely. After each increment of rotation, the force sensor measures the force required to maintain the end of the golf club shaft deflected to the holding point. As indicated above, measurements from the force sensor can be used to determine the location of the spine. Once the spine is located, the processor can be used to rotate the golf club shaft until the spine is oriented in the bend plane. For the present invention, the rotation of the golf club shaft to orient the spine in the bend plane can be performed while the shaft is deflected.
Once the spine is located and oriented in the bend plane, dynamic stiffness measurements can be performed. Specifically, the free end of the golf club shaft can be grabbed and deflected using the process described above. More specifically, the end of the golf club shaft can be deflected to a release point. Once the shaft is deflected, the hand can be reconfigured into the first configuration wherein the rollers are separated using the pneumatic cylinder to release the deflected end of the golf club shaft. Upon release, the end of the golf club shaft is free to oscillate about the tube axis. During oscillation, the oscillation frequency is measured using the photo eye and processor as described above. Additionally, the adjustable limit sensors can determine whether the golf club shaft oscillates outside of the bend plane by more than a predetermined amount. After dynamic stiffness measurements have been taken with the spine in the bend plane, the collets and golf club shaft can be rotated through a predetermined angle (such as 90 degrees) to a new orientation. After rotation to the new orientation, the golf club shaft can be deflected and released to thereby allow the oscillation frequency to be measured at the new orientation. This process can be repeated to allow the oscillation frequency to be measured at several angular orientations around the golf club shaft.